Spontaneous demixing of mixed active-passive suspensions

Understanding the properties of active matter is driving a rapid growth in soft- and bio-physics. Some of the most important examples of active matter are at the microscale, and include active colloids and suspensions of microorganisms, both as a simple active fluid and as mixed suspensions of active and passive elements. In the latter class, recent work has started to reveal new phenomena including activity-induced depletion interactions, phase separation, and the extraction of work from active suspensions. Building on current research in our group on the physics of colloid-swimmer interactions [1,2] we are interested in how external control of the dynamics of the active component can be used to alter the transport of passive cargo. Here we report our new experiments on the behaviour of active-passive systems under spatial confinement. Confinement-induced inhomogeneities in swimmers' distributions influence dramatically the spatial distribution of passive particles, with the colloids accumulating either towards the boundaries or towards the bulk of the sample depending on the size of the container. We show that this can be used to induce the system to de-mix spontaneously.

[1] R. Jeanneret, et al. Nat. Commun. 7, 12518 (2016).
[2] A. Mathijssen, et al. PR Fluids 3, 033103 (2018).